As is generally well recognized in the vehicle art, numerous vehicles utilized in the construction, mining and agricultural industries employ dual hydraulic braking circuits. A first of these braking circuits controls the brakes on the left-hand side of the vehicle and a second of these braking circuit controls the brakes on the right-hand side of such vehicle. These separate braking circuits are used in these vehicles for the purpose of steering or at least providing the operator assistance in steering such vehicle.
In some cases, these braking circuits may be rather high pressure circuits. Such high pressure circuits are usually supplied fluid pressure by a pump and/or hydraulic accumulator arrangement. In certain other cases, such as found in many of the agricultural vehicles presently in use, such braking circuits may consist of a relatively simple master cylinder/slave cylinder-type arrangement. In this latter case, it is generally the practice to utilize two independently operated master cylinders. In this case, each of the master cylinders is operated by its own separate brake pedal and controls its own brake circuit on a particular side of a vehicle. Therefore, these particular vehicles must be equipped with two brake pedals. Consequently, for the purpose of making a braking application on these vehicles, these two brake pedals are coupled together so that they will work in unison. On the other hand, for the purpose of steering, or providing assistance in the steering of such vehicle, these two brake pedals must be uncoupled and operated separately by the vehicle operator.
When these vehicles ar equipped with a trailer brake valve, which is operated by the vehicle's own brakes, it is the most desirable arrangement to have the trailer brake valve to operate only when the vehicle brakes are being used for the purpose of braking the vehicle. In other words, such trailer brake valve should not operate when the vehicle brakes are being used for the purpose of steering the vehicle. Therefore, as such, it is then necessary for such trailer brake valve to operate only when both the right-hand and left-hand brake circuits are in a pressurized condition.
Such a trailer brake valve, which uses control logic terms, is generally recognized in the valve art as an "AND" type trailer brake valve. Prior to the present invention, a number of different forms of "AND" type control logic valves have been both taught in the prior art and used in vehicle braking systems. One such well known AND-type control logic valve includes a pair of opposing non-return valves which have a push rod interspaced between them. This push rod has a sufficient length so that when either one of these non-return valves is in a seated position, i. e., a closed position, the other non-return valve will be held in an open position by such push rod. A quite similar type AND-type control valve can be constructed, however, without requiring the use of such push rod. In this latter AND-type control logic valve, there is a pair of opposing poppet elements. These poppet elements are either extended on their seats or are placed so close together that they will prevent both poppet elements from closing at substantially the sam time. In each case, however, the poppet element is made to close by the action of fluid pressure through the circumferential area between such poppet element and the seat. Therefore, the AND-type control valve is constructed to ensure that the fluid flow through this circumferential area is not too large. Consequently, this means that such circumferential area must be kept rather small. However, for master cylinder-type braking systems presently in use, the fluid flow required to close this AND-type logic valve represents a leakage flow and increased brake pedal travel and, therefore, must be kept to a minimum. In order to accomplish this, the circumferential area must be controlled to extremely tight tolerances. Such tight tolerances are unacceptable because they add significantly to both the rejection rate and the manufacturing cost of such AND-type control logic valve.